Starting of high temperature electrode lamps



y 5, 1966 E G. FRIDRKZH 3,

STARTING OF HIGH TEMPERATURE ELECTRODE LAMPS Filed May 9, 1963 lnvaw 1701- 1 ELmeT- QFfldTich His A t bor'ney United States Patent Office 3,259,778 Patented July 5, 1966 3,259,7 7 8 STARTING OF HIGH TEMPERATURE ELECTRODE LAMPS Elmer G. Fridrich, South Euclid, Ohio, assignor to General Electric Company, a corporation of New York Filed May 9, 1963, Ser. No. 279,153 4 Claims. (Cl. 313-217) This invention relates to the starting of electric discharge lamps of the kind having small tungsten electrodes operating with their tips either molten or close to the melting point in an iodine-containing discharge medium which provides electrode regeneration. The invention provides an improvement which reduces the starting voltage of such lamps.

The type of lamp in which the invention is most useful is described in my Patent 3,067,357, Electric Discharge- Lamp Electrode, and in my copending application Serial No. 108,773, filed May 9, 1961, entitled, Electric Discharge Lamp, and assigned to the same assignee as the present invention. Such lamps in general comprise a discharge envelope, suitably made of quartz, of small volume in order to have a high input concentration and an internal tube diameter preferably less than 1 centimeter. A pair of slender or rod-like tungsten electrodes are sealed into the ends of the envelope and are correlated in heat-dissipating capacity to the energy loading of the lamp to operate with their tips either molten or close to the melting point of the tungsten. The ionizable filling within the lamp includes an inert gas such as argon or xenon either iodine or a metallic iodide which releases iodine at high temperatures. The metal iodides which have been found most suitable for use in these lamps are those of indium, gallium and thallium. By suitable choice of filling, white light at efliciencies as high as 80 lumens per watt may be produced. Ordinarily the electrodes are operated with surface tension-supported molten ball points and the iodine vapor within the envelope serves to regenerate at the electrodes the tungsten which vaporizes from them.

A drawback with these l-amps is the high starting voltages which they require. For instance, a relatively short are gap lamp operating with a voltage drop of approximately 50 volts after it has warmed up, may require the application of an open-circuit voltage of several thousand volts to start or, alternatively, the application of circuit voltages as high as 500 volts plus high frequency starting pulses of several thousand volts. The hard starting characteristic of these lamps appears due to the low electron-emission of tungsten when cold.

Since the electrodes operate at or close to the melting point of tungsten, it has not been feasible to activate the electrodes. The activating material is vaporized off or else reacts with the iodine and deposits on the envelope walls and blackens them. At any rate it is no longer effective as an electron-emitter, nor will it reduce the starting voltage.

The object of the invention is to provide an improvement in such lamps which lowers the starting voltage and is compatible with the operating environment.

In accordance with the invention, I have found that the starting voltage may be substantially reduced by attaching a small amount of a refractory insulator to the shank of the electrode at an appreciable distance from the tip. At this place the temperatures are considerably lower and I have found that there are several insulating materials available which can withstand the iodine atmosphere of the lamp, thorium oxide or zirconium oxide being preferred for the purpose. In a preferred embodiment, the insulator is provided as a short sleeve around the shank of the electrode.

For a more complete explanation of the features and advantages of the invention, attention is now directed to the following description of a preferred embodiment to be read in conjunction with the accompanying drawing. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawing wherein like symbols denote corresponding parts throughout the several figures:

FIG. 1 illustrates a metal iodide compact source quartz arc lamp embodying the invention.

FIG. 2 is a sectional view to an enlarged scale through one end of the lamp.

The illustrated lamp 1 comprises a quartz envelope having a bulbous central portion or bulb 2 provided with generally cylindrical extensions 3, 4 referred to as necks. Electrodes 5, 6 consist of short lengths of tungsten wire which are welded to the foliated ends of molybdenum inleads 7, '8 extending through the neck, the foliated portions providing the hermetic seals. Each electrode includes a tapered front portion which attains its smallest diameter immediately before the ball points or spherical tips 9, 10. As taught in my earlier-mentioned patent, the lamp operates with substantially molten tips and the bulb or rounded ends on the electrodes may be formed after completion of the lamp by operating it at a current or loading sufficient to melt back the electrodes to the desired extent. The lamp contains an ionizable filling which includes an inert gas such as argon or xenon, and iodine or a metal iodide which releases iodine at high temperatures, suitably gallium iodide. By the iodine regenerative cycle, free iodine reacts with tungsten at the bulb wall to form tungsten iodide which is decomposed at the electrode with redeposition of the tungsten thereat and liberation of the iodine to start the cycle over again. This maintains the electrode stable in size and shape and eliminates envelope darkening by deposition of vaporized tungsten. 4

The illustrated lamp has no exhaust tip and may suitably be made from a one-piece bulb produced by the simultaneously shrinkage and upset of quartz tubing which is thereafter processed into a lamp by the tipless sealing and flush filling method described and claimed in my copending application Serial No. 279,270, filed of even date herewith, entitled, Electric Lam-p Manufacture, and assigned to the same assignee as the present invention.

In accordance with the invention, the starting voltage of the lamp is substantially reduced by applying to the shanks of the electrodes insulating sleeves 11, 12 of a refractory insulator. Suitably the sleeve may be a fired ceramic body of thorium oxide. Alternatively, the sleeve may be made of zirconium oxide. In general, the sleeve must be an insulator of very high melting point and nonreactive with iodine. The sleeve is slipped over the shank of the electrode prior to sealing into the bulb and may be retained in place by spot welding a small metal tab 13, 14 to the tungsten shank.

The insulating body need not necessarily be in the form of a sleeve as illustrated. For instance a small piece or sliver of thorium oxide may merely be attached to the shank or alternatively powdered thorium oxide or zirconium oxide may be fused or sintered as a layer around the electrode shank. The mechanism involved appears to be the increase in electric gradient due to the distortion in the electric field at the edge of the insulator. This promotes the emission of electrons and thereby facilitates starting. I have found that a lamp such as illustrated without a refractory insulator on the shank requires the application of a circuit voltage of approximately 500 volts coupled with an impressed high fre-- quency discharge of approximately 2000 volts in order to start. The provision of an insulator on the shank reduced the starting requirements to an impressed voltage of 400 volts without any need for an auxiliary high. frequency discharge.

Since the refractory insulator is on the shank of the electrode where the temperature is substantially lower than at the tip, it does not vaporize during operation of the lamp. Furthermore, since neither thorium oxide nor zirconium oxide are reactive with iodine even at the high temperatures involved, the refractory insulator successfully withstands the operating environment. Where the loading of the lamp is particularly high, it is advantageous to elongate the arc chamber into throat-like cavities extending into the necks of the envelope, as shown at 15 in FIG. 2. The refractory insulator 11 is located at root or end of the throat. Thereby the refractory insulator is more remote from the molten tip of the electrode and is not subjected to as high a temperature.

The illustrated lamp is intended for A.C. operation and therefore it is desirable to provide a refractory insulator on the shank of both electrodes. The requirement, however, is that the refractory insulator be placed on the shank of the cathode. Therefore on a lamp intended for DC. operation, only one such insulator need be provided, placed on the cathode.

The specific embodiment of the invention which has been illustrated and described is intended by Way of example only. Various modifications will readily occur and it is intended by the appended claims to cover any.

such falling within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge device comprising a sealed envelope of refractory insulating material, a pair of electrodes projecting into said envelope of which at least one is a small rod-like tungsten electrode serving as cathode correlated in heat-dissipating capacity to the energy loading of the lamp whereby to operate with its tip at a temperature close to the melting point of tungsten, an ionizable medium in said envelope including a substance providing iodine vapor in operation of the lamp, and refractory insulating material from the group consisting of thorium oxide and zirconium oxide on the shank of said one electrode and removed from the tip thereof to lower the starting voltage.

2. An electric discharge device comprising a sealed envelope of refractory insulating material, a pair of small roddike tungsten electrodes projecting into said envelope and correlated in heat-disspiating capacity to the energy loading of the lamp whereby to operate with their tips at a temperature close to the melting point of tungsten,

4 an ionizable medium in said envelope including a substance providing iodine vapor in operation of the lamp, and refractory insulating material from the group consisting of thorium oxide and zirconium oxide on the shanks of said electrodes and removed from the tips thereof to lower the starting voltage.

3. An electric discharge lamp comprising a sealed quartz envelope defining a discharge chamber with elongated throat cavities extending into neck portions of the envelope, a pair of tungsten electrodes sealed into said envelope and projecting through said throat cavities into said discharge chamber, an ionizable medium in said envelope including a substance supplying an atmosphere of iodine vapor in operation of the lamp, said electrodes being relatively small tungsten rods correlated in heatdissipating capacity to the energy loading of the lamp whereby to operate with their tips at temperatur s close to the melting point of tungsten, and insulating material more refractory than quartz and resistant to the attack of iodine vapor on the shanks of said electrodes and disposed Within said throat cavities for lowering the starting voltage of the lamp.

4. An electric discharge lamp comprising a sealed quartz envelope defining a discharge chamber with elongated throat cavities extending into neck portions of the envelope, a pair of tungsten electrodes sealed into said envelope and projecting through said throat cavities into said discharge chamber, an ionizable medium in said envelope including a substance supplying an atmosphere of iodine vapor in operation of the lamp, said electrodes being relatively small tungsten rods correlated in heatdissipating capacity to the energy loading of the lamp whereby to operate with their tips at temperatures close to the melting point of tungsten, and sleeves of refractory insulating material from the group consisting of thorium oxide and zirconium oxide around the shanks of said electrodes and disposed within said throat cavities for lowering the starting voltage of the lamp.

References Cited by the Examiner UNITED STATES PATENTS 1,930,088 10/1933. Foulke 3l3288 X 2,062,959 1/1936 Barclay 313-218 2,876,377 3/1959 Retzer 313-220 3,067,357 12/ 1962 Fridrich 313-223 3,140,417 7/1964 Tietze 313256 GEORGE N. WESTBY, Primary Examiner.

S. SCHLOSSER, Assistant Examiner. 

1. AN ELECTRIC DISCHARGE DEVICE COMPRISING A SEALED ENVELOPE OF REFRACTORY INSULATING MATERIAL, A PAIR OF ELECTRODES PROJECTING INTO SAID ENVELOPE OF WHICH AT LEAST ONE IS A SMALL ROD-LIKE TUNGSTEN ELECTRODE SERVING AS CATHODE CORRELATED IN HEAT-DISSIPATING CAPACITY OF THE ENERGY LOADING OF THE LAMP WHEREBY TO OPERATE WITH ITS TIP AT A TEMPERATURE CLOSE TO THE MELTING POINT OF TUNGSTEN, AN IONIZABLE MEDIUM IN SAID ENVELOPE INCLUDING A SUBSTANCE PROVIDING IODINE VAPOR IN OPERATION OF THE LAMP, AND REFRACTORY INSULATING MATERIAL FROM THE GROUP CONSISTING OF THORIUM OXIDE AND ZIRCONIUM OXIDE ON THE SHANK OF SAID ONE ELECTRODE AND REMOVED FROM THE TIP THEREOF TO LOWER THE STARTING VOLTAGE. 